The Traffic/Alert collision Avoidance System (TCAS) located aboard a protected aircraft periodically transmits interrogation signals which are received by Transponders located aboard other aircraft, hereinafter referred to as target aircraft, in the vicinity of the protected aircraft. In reply to the interrogation signals, the target aircraft's transponder transmits a response signal. The TCAS equipment aboard the protected aircraft determines the range of the target aircraft in accordance with the round trip time between transmission on the interrogation signal and receipt of the response signal. Relative bearing to the target aircraft is determined from differences in the time to different elements in the TCAS antenna.
The Transponder is an airborne receiver-transmitter portion of air traffic control Radar Beacon System mode A and mode C interrogations as well as mode S interrogations. The Transponder sends an identifying coded signal in response to a received interrogation from a ground-based radar station to locate and identify the aircraft. Reply signals from the transponder are used to generate displays of the replying aircraft identification, position and altitude for air traffic controllers. The mode S function of the Transponder is used to transmit TCAS-related information between TCAS-equipped aircraft.
Generally TCAS and Transponder systems employ separate antennas and transmitters. Thus, each aircraft may include a top and bottom TCAS antenna and a top and bottom transponder antenna. Additionally, a separate cable is required for each antenna. Referring to FIG. 1, conventional separate aircraft TCAS and Transponder systems 100 are shown. TCAS top antenna 101 and bottom antenna 102 are connected through cables 103, 104 to a TCAS transmit/receive block 108, which includes TCAS switches 105, TCAS transmitter 106, TCAS receiver 107. Transponder top antenna 109 and bottom antenna 110 are connected through cables 111, 112 to a Transponder transmitter/receiver block 115, which includes Transponder switches 113, Transponder transmit/receive 114. The TCAS transmit/receive block 108 is connected to Transponder transmit/receive block 115 by communication link 116 and suppression line 117. This suppression feature prevents interference by other equipment, such as distance measuring equipment (DME). Conventional separate TCAS and Transponder systems require at least four antennas, separate cables and separate transmitters. Therefore these systems are heavy, occupy a substantial amount of space and are very costly.
These disadvantages of separate TCAS and Transponder systems may be eliminated with a combined TCAS/Transponder with a common antenna system and a common transmitter. A multifunctional aircraft transponder is disclosed in U.S. Pat. No. 6,222,480. FIG. 2 illustrates a combined TCAS Transponder system 200 as disclosed in U.S. Pat. No. 6,222,480. Switch 205 connects the Transponder receivers 208, the TCAS receivers 207, and a combined TCAS/Transponder transmitter 206 to antennas 201 and 202 through cables 203 and 204. The Transponder receive function of this system utilizes the directional (left, right, fore or aft) and location information provided by antennas 201 and 202 to determine the directional source of an interrogation signal. Such capability is available in a conventional four-element directional antenna as is used in current TCAS systems. The Transponder receivers 208 determines the relative vertical direction of a received interrogation signal based on difference in signal strength of and determines azimuth bearing from comparison of the relative signal strength at the four elements of top antenna 201 or bottom antenna 202.
The quantity of Transponder receivers is equal to the total quantity of monopoles of both top and bottom antennas. Therefore, this system includes eight transponder receivers (four receivers for the top four-element antenna and four receivers for the bottom four-element antenna). Each typical transponder radio frequency (RF) receiver includes a variable phase shifter (in the phase system), preselector (high Q-factor first band-pass filter, low noise amplifier, and second band-pass filter), limiter, mixer, etc. Eight of these complicated Transponder RF receivers are costly, and also have high weight and occupy a substantial amount of space.
Additionally, the multifunctional aircraft transponder known to the art includes two different systems to form directional antenna pattern: phase interferometric bearing estimation and amplitude comparing bearing estimation. The main condition for high-efficiency combining of all Transponder receiver channels is the equality of different signal's phases. The phase and amplitude differences include errors relating to transmission path difference between the channels and antenna cable errors. To eliminate these errors, a special calibration network with variable phase shifters, phase detectors, additional calibration signal source is required.
The typical Transponder receiver receives 1030 MHz signals and the TCAS receiver provides receiving of 1090 MHz signals. The multifunctional aircraft transponder including connection of all receivers with common antennas through the switch does not provide selection of above signals and has additional losses for input receive signals. Therefore, this multifunctional aircraft Transponder is expensive, has extra size and weight, and poor electrical performance.
Consequently, a combined TCAS/Transponder RF transmitter/receiver with common antenna system that provides improved performance over separate TCAS and Transponder systems while solving the attendant problems of a combined TCAS/Transponder.